Always a an interesting topic because if I did make something I could be liable to be sued. It's one of the reasons that big studios will not take ideas, and movie studios do not read scripts without a contract.

That said if you genuinely want to part ownership of any ideas I'm always keen to talk games design. Although I do have about 5 games already queued up in my head to make. :p

Well I have one I think is geniune good but I understand the "liable for lawsuit" point. Eventhough you're in the UK and I'm in the netherlands and I believe there isn't a big "lawsuit" culture like in the US.

So here is another idea,
Peter Molyneux like game idea:

It's a god game only you are an alien species discovering earth around 3000 BC and the game takes place till around 2200 AD or something.
Catch is you can only influence the people, not control them directly and they can't know for sure aliens are out there.
This ties in with those funny creazy theories that maya's and inca's met with aliens and they gave them "technology". The piramids were build by aliens etc.
The tric of the game is that in earlier times people will believe you are god or a god but in later times, when things are recorderd you have to be more carefull. And in present day or future day where everyone has cell phones and camera's and wordl wide news you have to find ways to influence mankind without being exposed.
Maybe your goal is to help mankind to get to where other galactic species technologies are, so they humans can join galactic council blablabla
Or maybe your species need earth's water/energy/life force something but to do that you need humans to help you regain that.

You can buy it for 1 euro so you won't get sued by me
good luck
sorry for grammar/spelling mistakes

Paranoia fps or top down surival game, where every game the imitated versions of your base companions is randomised so you can't figure out who the thing is (and go back to an old save and just shoot them.)
There is a paranoia gage
there is a "others suspect you" gage
win condition: make sure the thing can't reach civilization and/or kill all the things. Don't kill innocents. Complete objective in 3 days or freeze to death.
Enemy AI will try to get alone time with real humans.
Enemy AI will sabotage the base lights, heating, water etc.

Paranoia fps or top down surival game, where every game the imitated versions of your base companions is randomised so you can't figure out who the thing is (and go back to an old save and just shoot them.)
There is a paranoia gage
there is a "others suspect you" gage
win condition: make sure the thing can't reach civilization and/or kill all the things. Don't kill innocents. Complete objective in 3 days or freeze to death.
Enemy AI will try to get alone time with real humans.
Enemy AI will sabotage the base lights, heating, water etc.

Could even make this in the maia game engine

Hmm would Love a proper "The Thing" esk experience in a game. Thou I'd personally go "The ship" route and have it all the NPC's are players, kinda like Murder/werewolf party game. 30min - 1 hour long game, Random player at the start is "The Thing" and has to sabotage/infect the others, while everyone else has to work out who it is.

Might make for a fun LAN party game, or suck because no one can find a good group ^^